The present invention relates to a brake fluid pressure control device of simple structure and having the functions of both antilock and traction control.
The simplest anitlock device known is of the so-called recirculating type, which comprises a wheel brake fluid pressure control valve having a discharge valve and provided in a main fluid passage extending from the master cylinder to each wheel brake, a discharged fluid reservoir for temporarily storing the brake fluid discharged from the discharge valve, and a pump for pumping the brake fluid out of the discharged fluid reservoir and returning it to the main fluid passage. If it is desired to add traction a control function to this device, the structure of the entire device can be simplified effectively by using a circulation pump for the antilock function as a pressure source for the traction control.
In order to employ such a structure, brake fluid has to be supplied to the inlet port of the pump. This can be done in several known ways. These known ways are classified into two types. In one type, a fluid supply passage is provided so as to extend directly from the reservoir for the master cylinder to the pump inlet port. In the other type, the supply passage is provided to branch from the main passage connecting the master cylinder outlet port to each wheel brake. Of these two types, the latter is advantageous in view of easy mounting on a vehicle and the non-necessity of returning redundant brake fluid to the reservoir for the master cylinder through its inlet port.
The latter method, in which the fluid supply passage branches from the main fluid passage, requires a traction control changeover valve for checking fluid flow from the flow return point from the pump toward the master cylinder during traction control and a shutoff valve for checking fluid flow from the fluid supply passage toward the discharged fluid reservoir during normal braking (when the master cylinder pressure is applied). As the shutoff valve, a solenoid valve or a pressure-responsive valve is ordinarily used.
But a solenoid valve is not only relatively expensive, but also has a problem in that, if the valve were not returned to its inoperative position as soon as the brake pedal is depressed during traction control, the brake fluid in the master cylinder would flow into the discharged fluid reservoir, thereby making the brake pedal inoperative. In this state, the subsequent antilock control function may be lost, too. Thus, it is necessary to increase the reliability of the electric system, including a brake switch. This is costlier than it appears.
A pressure-responsive valve, which requires a cylinder/piston assembly for driving the valve body in response to the pressure, is never satisfactory in cost.
In order to solve these problems, the present applicant proposed in Unexamined Japanese Patent Publication 5-116556 a brake fluid pressure control device of the above-described type which is simple in structure and inexpensive. It has a shutoff valve in the form of a stroke-responsive type check valve which is operative to check the fluid flow from the fluid supply passage to the discharged fluid reservoir if the amount of brake fluid in the discharged fluid reservoir exceeds a predetermined level. In this arrangement, the piston for the discharged fluid reservoir, which is an indispensable part in this structure, is used to drive the valve body of the check valve.
Since the piston in the discharged fluid reservoir, which is an essential part in this structure, is used to actuate the stroke-responsive check valve, such a shutoff valve is fairly simple in structure and thus costs less. Because of its simple structure, it is more reliable in operation than solenoid valves or pressure-responsive valves. But the stroke-responsive check-valve disclosed in Unexamined Japanese Patent Publication 5-116556 has a problem in that it necessarily has to be installed in or near the discharged fluid reservoir. If it is desired to add the traction control function to only some of mass-produced antilock control devices originally having no traction control function by adding the stroke-responsive check valve, such devices have to be provided with an extra space for mounting the check valve near the discharged fluid reservoir when mass-producing them. This limits the freedom of layout and reduces the economic advantage.